Direct reduction plants for producing direct reduced iron, known as DRI or sponge iron, hot briquetted iron, or the like (in general prereduced materials useful as feedstockcs for iron and steelmaking), currently produce it by reacting a reducing gas, at temperatures in the range of 750.degree. to 1050.degree. C. and composed principally of hydrogen and carbon monoxide, with a bed of particulate iron-containing material in the form of lumps or pellets. Nowadays, most operating direct reduction plants utilize a moving bed reactor where the gases flow countercurrently to a gravity descending bed of iron ore particles flowing down through the reactor. Examples of such processes are described in U.S. Pat. Nos. 3,749,386; 3,764,123; 3,816,101; 4,002,422; 4,046,557; 4,336,063; 4,375,983; 4,428,072; 4,556,417; and 5,078,787.
It is known that DRI used as a charge or part of the charge to a steelmaking electric arc furnace (EAF) should desirably contain certain amounts of carbon chemically combined with the iron in the DRI material. Combined carbon, in contrast to the free carbon which can be added to the molten iron bath in the EAF, as soot in the DRI or as graphite, provides a number of advantages to the steelmaking process, e.g., a larger proportion of said carbon (about 70 to 85%) remains in the liquid iron bath and contributes to further reduction of the iron oxides contained in the DRI feedstock forming carbon monoxide; the reduction reactions of this carbon monoxide produce gas bubbles which form a "foamy" slag layer over the molten iron bath with its much appreciated effects on the protection of the EAF walls against the radiation of the electric arc; the carbon monoxide also provides energy to the EAF when further oxidized to carbon dioxide, thus saving in electric energy consumption.
It has long been desired to have a direct reduction process wherein the DRI product contains the right amount of chemically combined carbon which best fits the particular characteristics of a steelmaking operation.
The currently operating reduction processes produce DRI with an amount of carbon within a certain narrow range, between 0.8% to 1.8% depending on the average composition of the reducing gas, because the carburization is affected principally by the Boudouard's reaction: 2CO.fwdarw.C+CO.sub.2. This reaction is exothermic and is promoted at relatively low temperatures, i.e., in the range of 500.degree. C. to 700.degree. C.; consequently, it has been customary to promote this carburization reaction in those processes wherein the DRI product is cooled down to ambient temperatures before being discharged from the reduction reactor by circulating a CO-containing gas through the cooling-discharge portion of said reactor.
Another way of obtaining a DRI product with a desired amount of carbon is to contact the hot DRI product with natural gas in the cooling zone of the reactor. The hydrocarbons in natural gas, as exemplified by methane, crack to elemental carbon which combines with the metallic iron and hydrogen plus carbon monoxide, utilized in the reduction zone. This is a well-known practice, as shown, for example, in U.S. Pat. Nos. 4,046,557 and 4,054,444, which latter patent also proposes to feed the carburizing natural gas to the intermediate zone between the reduction zone and the cooling zone of the reduction reactor in order to utilize the heat of the DRI for cracking the hydrocarbons.
The cracking reaction involved is CH.sub.4 .fwdarw.C+2H.sub.2. Since cracking of hydrocarbons is strongly endothermic, this reaction is mostly used in those processes producing "cold" DRI product. Due to the above reaction, natural gas has been used as a coolant in some processes, for example, in U.S. Pat. Nos. 3,765,872 and 5,437,708. This latter patent discloses a process wherein the amount of carbon in the DRI is increased by extending the residence time of the DRI produced in the reaction zone. This method, however, is not practical because the residence time is increased from 5 to 6 hours to 9 to 15 hours. This requires a larger reactor in order to have the same production rate.
Fine and reliable control of carburization of DRI becomes somewhat more difficult when the DRI product is conveniently discharged from the reduction reactor at high temperatures (i.e., above 550.degree. C.) for immediate utilization in an EAF with great advantages in energy savings and productivity of the steelmaking process, or for production of hot briquetted iron (HBI) with its advantages, its transportation by land or sea and its use in the steelmaking furnace. There have been some proposals for achieving the desired amount of carbon combination in a process producing hot DRI. One such method is described in U.S. Pat. Nos. 4,834,792 and 4,734,128. These patents describe processes where a reducing gas with a predetermined reducing power is produced in a separate reformation furnace, where hydrocarbons in natural gas are converted to H.sub.2 and CO in the reforming furnace and the carburizing hydrocarbons are added to the reducing gas fed to the reactor.
Another proposal for producing "hot" DRI with a high amount of carbon is disclosed in German OS 44 37 679 Al wherein natural gas is fed to the discharging portion of the reduction reactor in order to crack the hydrocarbons utilizing the heat of the DRI flowing down from the reduction zone. This method of carburization is the same as described above with the only difference that the reducing gas is produced within the reactor. This patent, however, presents the drawback that again the amount of energy available for carrying out the endothermic carburization reactions is the heat of the DRI; if the DRI is to be discharged at high temperature, the amount of carburization will be very limited.
The present invention is an improvement over the prior art processes and particularly discloses an improvement over U.S. Pat. No. 5,110,350 to Villarreal-Trevino et al. This patent describes a direct reduction process without an external natural gas reformer, wherein the reducing gases are generated by reformation of natural gas with water, added to the reducing gas before the reducing gas stream is heated, by saturating it with hot water taken from the off gas cooler. The mixture of natural gas water and recycled gas is heated in a gas heater and fed into the reduction reactor wherein the reformation reactions, the reduction reactions and the carburization reactions, all take place therein. This patent, however, does not utilize oxygen for partial combustion of the reducing gas before this gas is fed to the reduction reactor in order to provide the energy necessary for carburization of the DRI to the desired predetermined degree.
Other patents which are related prior art are U.S. Pat. No. 3,375,099 to W. E. Marshall which discloses a reduction process for iron oxides wherein natural gas or methane is partially combusted with oxygen in a combustion chamber for production of hydrogen and carbon monoxide in a known manner. Only a minor portion of the regenerated gas can be recycled to the reactor because the temperature of the gas entering the reactor would fall down excessively since no gas heater for the recycled stream is provided. The consumption of fresh natural gas is therefore large and valuable reducing gas must be wasted because of this limitation. The consumption of oxygen is also high because all the heating necessary for raising the temperature of the reducing gas to the reduction level must be provided by partial combustion of the natural gas with oxygen.
U.S. Pat. No. 5,064,467 to Dam et al. discloses a direct reduction process similar to that of the German OS 4437679 wherein the reducing gases are produced by partial combustion of a mixture of recycled gas and natural gas with air or air plus oxygen whereby the hydrocarbons of the natural gas are reformed within the reduction reactor as known in the art. This process, however, does not take advantage of a good level of humidity for reformation of the natural gas but relies on carbon dioxide and oxygen for reformation. Since this process does not comprise a CO.sub.2 removal unit for regenerating the recycled gas, the amount of gas exported from this system is in the order of 30% of the gas effluent from the reactor.
U.S. Pat. No. 4,528,030 to Martinez Vera et al. discloses a reduction process without an external reformer where natural gas is reformed with steam as the main oxidant within the reduction reactor. This patent, however, does not include addition of oxygen for increasing the temperature of the reducing gas entering the reactor and for providing the energy necessary for carburization of the DRI and the flexibility for controlling the amount of carbon control as in the present invention.